(a) Field of the Invention
The present invention relates to a method for preparing a high molecular weight polycarbonate resin, and more particularly to a method for preparing a high molecular weight polycarbonate resin, which can produce a high molecular weight polycarbonate resin that has narrow molecular weight distribution and this has uniform physical properties, and that can be used for injection and extrusion, in large quantities with a low cost within a short time.
(b) Description of the Related Art
As polycarbonate resins have excellent heat resistance, impact resistance, mechanical strength, transparency, etc., they are widely used in the preparation of compact disks, transparent sheets, package wrapping, car bumpers, films for UV blockers, and so on, and demands therefore are rapidly increasing.
The previous processes for the production of polycarbonate resins can be divided into interfacial polymerization using a phosgene, and melt polymerization and solid state polymerization that do not use a phosgene.
Interfacial polymerization is a process carried out by mixing an aqueous solution of aromatic hydroxy compounds such as bisphenol A and a gaseous state phosgene in an organic solvent to allow a polymerization reaction to occur in the interface between the aqueous solution layer and the organic solvent layer.
This process enables the production of high molecular weight polycarbonates in a comparatively easy manner by a continuous process, but as it uses toxic gases and chlorine-type organic solvents that are noxious substances, it has a very high risk and therefore it requires excessive facility costs.
Melt polymerization, a process wherein the polymerization proceeds using starting monomers in their melt state, has the advantage of having a low risk because it does not use toxic materials, but in order to produce high molecular weight polycarbonates for injection and extrusion, a high temperature, high vacuum facility is needed when high viscous reactants are treated, and thus the quality is deteriorated. Solid state polymerization is a process wherein the polymerization proceeds at a temperature lower than the melting temperature after the crystallization of low molecular weight polycarbonate prepolymers, it does not use toxic materials, and it can prevent the deterioration of quality because the reaction occurs in a solid state.
In the solid state polymerization disclosed in U.S. Pat. No. 4,948,871 and U.S. Pat. No. 5,214,073, a crystalline polycarbonate, which can be polymerized in a solid state manner, was prepared by heat treatment of a non-crystalline polycarbonate prepolymer at its crystallization temperature over a long time (crystallization system by heat treatment) or by dissolving it in a solvent and precipitating it with a non-solvent (non-solvent precipitation method).
In the crystallization system by heat treatment, a long time (72 hours or longer) is required and the degree of crystallization is comparatively low, and thus it is difficult to prepare a crystalline polycarbonate suitable for solid state polymerization.
In the crystallization system by non-solvent precipitation, as two kinds of solvents are used, these solvents should be completely separated after crystallization, and the crystallization state between early and end processes may be different due to the ratio difference between the solvent and non-solvent during the precipitation.
The solid state polymerization disclosed in U.S. Pat. No. 6,031,063 and U.S. Pat. No. 6,222,001 employed a crystallization system that was conducted by contacting a non-crystalline polycarbonate in a pellet state with a difficult solvent in a gas or liquid state at its crystallization temperature for a certain time. This system enables the preparation of a crystalline polycarbonate suitable for solid state polymerization within a comparatively short time, but it has a problem of requiring pelletization or pulverization before crystallization.
Therefore, in order to prepare crystalline polycarbonates necessary for solid state polymerization in an economical way, studies about excluding or simplifying the pulverization and drying steps in the previous crystallization processes are needed, and in addition, so as to minimize the time required for solid state polymerization, it is essential to develop a crystallization process capable of preparing crystalline polycarbonates that are the most suitable for solid state polymerization by controlling the size and distribution of crystalline polycarbonates.
The solid state polymerizations disclosed in U.S. Pat. No. 4,948,871, U.S. Pat. No. 5,214,073, U.S. Pat. No. 5,717,056, and U.S. Pat. No. 5,905,135 comprise the following three stages:
Stage 1: Preparation of a low molecular weight polycarbonate prepolymer by an ester-exchange reaction of a dialkyl(aryl)carbonate and an aromatic hydroxy compound;
Stage 2: Preparation of a crystalline polycarbonate by the crystallization of the low molecular weight polycarbonate prepolymer of stage 1; and
Stage 3: Preparation of a high molecular weight polycarbonate by the solid state polymerization of the crystalline polycarbonate of stage 2.
However, these methods are also problematic because the maximum molecular weight reaches at most 38,800 g/mol after more than 8 hours of solid state polymerization due to the large molar ratio difference between the arylcarbonate groups and aromatic hydroxy groups due to the reaction byproducts and unreacted diarylcarbonates that co-exist together with the low molecular weight non-crystalline prepolymer.
In addition, as such molar ratio difference is supposed to be increased in proportion to the increase in the molecular weight of the prepolymer to enhance the properties of the polycarbonate, it has an adverse effect on the increase of the molecular weight in the solid state polymerization for the same reason as above even though it may be advantageous to properties. Further, in order to carry out the solid state polymerization, non-crystalline polycarbonates should be converted into crystalline polycarbonates, and for this, in the above preparation methods, a separate crystallization process is used before the solid state polymerization. As examples of the crystallization process in the above processes, there can be included a method of treating a non-crystalline polycarbonate prepolymer with heat at its crystallization temperature for a long time, and a method for preparing a crystalline polycarbonate that can be polymerized in the solid state by precipitating a non-crystalline polycarbonate with a non-solvent after dissolving it in a solvent.
In the crystallization system by heat treatment, a long time (72 hours or longer) is required and the degree of crystallization is comparatively low, and thus it is difficult to prepare a crystalline polycarbonate suitable for solid state polymerization.
In the crystallization system by non-solvent precipitation, as two kinds of solvents are used, these solvents should be completely separated after crystallization, and the crystallization state between early and end processes may be different due to the ratio difference between the solvent and non-solvent during the precipitation.
Therefore, in order to prepare high molecular weight polycarbonates through solid state polymerization, it is necessary to substantially reduce the reaction time by maximizing the increase rate in molecular weight during the solid state polymerization, and to simplify the overall manufacturing process by improving the main process stage that is divided into crystallization and solid state polymerization, as well as additional process steps such as a separate pulverization step, a drying step, etc. It is essential that the properties of the high molecular weight polycarbonates to be produced through the solid state polymerization are uniform by efficiently controlling the distribution, size, and crystallinity of the polycarbonates used in the solid state polymerization.